Vaporization cooled electrical apparatus such as transformers usually utilize a pump as shown in FIG. 1 to spray a vaporizable dielectric fluid 5 upon the electrical windings 6 enclosed in a tank 4 to remove the heat generated through vaporization of the fluid 5. The vapor is condensed by a heat exchanger 7 and the condensate flows to a sump 8 and is recirculated to the windings 6 by the pump 3. Since only small volumes of liquid are required and since the vapor provides dielectric strength, any leaks of fluid can lead to transformer failure or shutdown, due to overheating or reduced dielectric strength. Since vaporizable fluids are used, small leaks are not visible to the eye due to evaporation. Conventional pumps with shaft seals will inherently wear out and leak during the 20 to 40 year expected life of such apparatus. For these reasons, a pump without a shaft seal or stuffing box is desirable.
Large electrical apparatus, such as transformers, utilize welded steel tanks which inherently have small quantities of metallic particles from the manufacturing processes remaining in the tank and piping. In transformers utilizing natural convection of a dielectric grade of mineral oil for cooling, oil velocities are very low so that these particles will not be moved from their resting positions in crevices and tank bottoms. However, in vaporization cooled transformers the fluid velocities are very high and these particles are circulated through the piping. In addition, the spray pattern and solvent action of the fluids used cause small particles of insulation to be dislodged and washed into the circulation system. If these particles enter the pump bearings, excessive wear and possible pump failures can occur which lead to failure or shutdown of the transformer.
One solution to this problem is to use a strainer or filter in the pump suction line. This solution, however, has several disadvantages. In order to remove small particles, a fine mesh filter must be used. This element introduces an additional pressure drop in the suction line which the pump suction pressure must overcome. Since the fluids are very volatile, this additional suction by the pump intake can lower the fluid pressure below the vapor pressure, causing the generation and collapse of vapor bubbles in the fluid known as "cavitation." This reduces the flow rate and leads to wear of the impeller, scroll, and bearings. In addition, the intake filter can become clogged which reduces the pump flow and may lead to overheating of the transformer unless the unit is shut down and the filter changed. The problem is not as detrimental if the strainer is in the discharge line where the pump discharge pressure is several times larger than the suction pressure; however, this allows particles to circulate through the pump prior to entering the strainer.
For the above reasons, it is desirable to provide a pump design in which a suction line strainer is not required and wherein the bearings are not exposed to the particles in the fluid, and wherein seals which wear and allow fluid leakage to the atmosphere are not required.
U.S. Pat. No. 3,138,105 discloses one pump (FIG. 2) which does not require shaft seals. A stainless steel shell 10 is inserted into the annular gap between the motor rotor 11 and the motor stator 12 to seal the stator 12 from the flow of the fluid. Another stainless steel shell 13 is inserted into the gap to completely enclose the rotor 11 and prevent contact of the fluid with the rotor 11. This pump is commonly called a "canned rotor" pump. A portion of the fluid pumped is through circulated by the impeller 14 through a screen assembly 15, holes in the front bearing 16, through the annular gap between the shells 10 and 13 and through holes in the back bearing 17. The fluid returns to the scroll 18 through a hollow shaft 19. The fluid pumped serves to lubricate the bearings 16,17 and cool the motor rotor 11 and stator 12. Heat is also transferred from the stator 12 by air convection from outer shell 20 which is not air tight. The bearings 16,17 are usually made from graphite.
The prior art "canned rotor" pump is especially suitable for pumping corrosive chemicals by preventing contact of the fluid with the rotor 11 and stator 12. However, the prior art "canned rotor" pump was found to be unsuitable for use on vaporization cooled transformers containing small particles in the fluid. The particles in the fluid pass through the clearances between the screen 15 and shaft 19. These particles are forced by the fluid circulation into the bearings 16,17 and into the gap between the stator shell 10 and the rotor shell 13. The particles cause excessive wear of bearings 16,17 and shaft 19 which causes the rotor shell 11 to contact the stator shell 10. Continued operation of the pump causes a hole to wear in the stator shell 10 which allows the fluid to escape, causing electrical failure of the apparatus or shutdown for pump repair. The temperature of the stator 12 is high since the efficiency of air cooling is poor. The stainless steel shells 10,13 in the gap between the rotor 11 and the stator 12 introduces additional thermal impedance to the heat flow of the pumped fluid. The stainless steel shells 10,13 in the magnetic field also increase the amount of electrical losses and cause additional heating. Since the volume of fluid between the rotor shell 13 and the stator shell 10 is small, the fluid can vaporize when volatile fluids are pumped. The fluid vapor enters the scroll 18 and causes loss of flow and overheating of the stator 12. The graphite commonly used for the bearings 16,17 is also a possible contaminant of dielectric fluids.
Other prior art inventions for pumping vaporizable dielectric fluids include vapor push pumps are disclosed in U.S. Pat. Nos. 3,819,301 and 3,834,835. These type pumps require large amounts of electric power to heat the fluid so that the efficiency is low.
It is an object of this invention to provide a pump for conveying vaporizable dielectric fluids containing solid particles by preventing entrance of these particles into the bearings and without rotary shaft seals which may cause leakage of the fluid to the atmosphere.